Why should I play this class. . .Warlock?

The Warlock. Probably one of the most hotly contested classes since the inception of Warcraft, for the majority of the time the game has been in existance, they have been called overpowered. They are. They are one of the few classes in Warcraft with few weaknesses, although unlike the Paladin, they are restricted to one role only. And they also benefit from a free mount.

Solo:
Strengths: The Warlock has a number of strengths. First of all, they are one of only three classes to employ a pet, which if used correctly allows the user plenty of time to get prepared while the pet takes damage. They offer a number of abilities which reduce downtime being able to drain both health and mana, and can convert their own health into mana. They have multiple fear abilities if situations get too difficult. Lastly they have the ability to store their souls and basically get a free resurrection. The Warlock is undoubtedly one of the most powerful classes in the game in solo play.

Weaknesses: Looking for solo weaknesses in the warlock is like looking for blemishes on a supermodel. They're there, but they're hard to find. They can't AOE as well as a mage and multiple mob pulls could become a problem. They are casters so are restricted to cloth armor and although they normally have the highest stamina of any caster, are squishy compared to the non caster classes.

Group:
Strengths: Soul stone is great for making sure the caster can rez if the group gets killed. They have strong sustained damage and AOE abilities. They also have very strong CC abilities, depending on the mobs being faced and the pet the Warlock is employing.

Weaknesses: Again, few to speak of. Fear if used at inappropriate times can be a nightmare for groups. Can pull aggro if it is not managed effectively. CC may not be usable at all times, depending on mob types.

PvP:
Strengths: DoT, DoT, DoT, DoT, Fear, repeat. It sucks, its cheap, anyone who's ever pvp'd has been hit by it, but it is effective. Oh, and the succubus can charm you. And they have an instant fear that ALSO drains life. Yeah, they're still overpowered in pvp.

Weaknesses: Depending on spec, they lack burst damage. They also are cloth wearers and a few good hits will take them out. If you can resist their fear effects, you can lay the hurt on them.

Specs:
Affliction: This is the main PvP spec for Warlocks. It increases abilities to apply DoTs on targets and drain health. This gives affliction buids very little downtime as they can convert health to mana, then drain the health from mobs (this is what is commonly referred to as "drain tanking"). This build also offers buffs to the already powerful seed of corruption, one of the more powerful AOE spells in the game if used correctly.

Demonology: Strong pets = strong warlock. This spec increases the effectiveness of the warlock's pets. It also offers a unique summon: the felguard. The felguard is like having a DPS warrior duoing with the warlock, except it is under control of the warlock and never rolls for loot. The only drawback of this line is the 51 point talent. Although turning into a demon is cool in concept, the power has a lot of weaknesses and another summon (*cough* Doomguard *cough*) would have been more appropriate.

Destruction: sometimes, you just want to blow sh*t up. For those times, spec destro! Destro locks produce excellent DPS, being slightly more bursty than the other warlock varieties. It is what it is, pure destructive spell damage. Its the "if brute force doesnt work, you're just not using enough" theory of casting. The drawback of this line is that it suffers from slightly more down time than the other two lines.

If you ever thought "I really have no desire to heal or tank, I just want to level and PvP" the warlock is a very solid choice. They are the most durable casters in the game, produce solid DPS in any spec and offer very solid utility in solo PvE, group PvE, and PvP.